1. Field of the Invention
The present invention relates to a method for manufacturing a bearing material coated slide member and to the bearing material coated slide member, and more particularly to a method for manufacturing a bearing material coated slide member in which a bearing material is coated on a sliding surface of a slide member and to the bearing material coated slide member manufactured by the method.
2. Description of the Related Art
A sliding surface of a slide member, such as a connecting rod used for a vehicle engine, is provided with a bearing, such as a sleeve bearing, to reduce friction between the sliding surface and a shaft acting as a rotation axis, thereby enhancing lubricity to improve the sliding property of the sliding surface. As a bearing material used for forming the bearing, a so-called white metal, such as an Sn—Sb—Cu alloy or a Pb—Sn—Sb alloy, a Cu—Pb alloy, a tin bronze, a lead bronze, an aluminum alloy, or the like is used. Because the alloys are generally soft materials, the alloys exhibit properties such as anti-seizing, fitting to a shaft, and the like. Such bearings are typically manufactured by casting or sintering one or more of the above-described alloys, and are used in a state in which the bearings are fitted into the sliding surface of the slide member.
FIG. 11 schematically shows a metallographic structure of a bearing composed of an Al—Sn—Si alloy, which is an example of aluminum alloys manufactured by casting and subsequent rolling of a material. The metallographic structure of the Al—Sn—Si alloy exhibits an organization in which Sn is finely dispersed in a base material of Al. Such fine dispersion of Sn imparts anti-seizing and other properties to the bearing. In addition, the metallographic structure of the Al—Sn—Si alloy exhibits uniform precipitation of Si. The uniformly precipitated Si grinds a shaft which is an object to be fitted, which yields improvement in wear resistance of the bearing while strengthening the property of preventing the bearing from seizing to the shaft.
In addition to the above-described casting and sintering, methods for directly forming a coating for use in bearing on a sliding surface of a slide member are employed for manufacturing a bearing. Methods for directly forming the bearing on the sliding surface of the slide member includes physical vapor deposition (PVD), thermal spraying, and others. PVD is a method for evaporating a metallic material, a ceramic material, or the like to physically coat the evaporated metallic material on a base material, and the method includes ion plating, sputtering, and vacuum evaporation. PVD is advantageous in that it provides a capability of forming an elaborate coating on a sliding surface of a slide member, but has a disadvantage in that productivity is relatively low due to the slow deposition rate in formation of the coating. Accordingly, PVD is not suited to mass production. Further, because a coating formed by PVD generally has a high hardness, use of the coating can lead to deterioration of anti-seizing property due to the hardness of the coating.
Thermal spraying is a method in which a thermal spray material, such as a metallic material or a ceramic material, is fused and sprayed onto a base material by means of high pressure gas to form a coating. Thermal spraying includes oxy-fuel spraying, such as flame spraying in which a thermal spray material fused in a high-temperature combustion gas is sprayed, and electric spraying, such as plasma spraying in which the thermal spray material fused in a plasma jet is sprayed. Because, in the thermal spraying, a metallic material, a ceramic material, or the like is fused to form a bearing on a sliding surface of a slide member, a deposition rate in formation of coating by the thermal spraying is advantageously faster than that using the PVD method. Accordingly, thermal spraying is generally employed when the bearing is directly formed on the sliding surface of the slide member.
FIG. 12 schematically shows a metallographic structure of an Al—Sn—Si alloy powder being a thermal spray material used for manufacturing a bearing composed of an Al—Sn—Si alloy by thermal spraying. Si exhibits a structure dissolved in Al, whereas Sn exhibits a structure finely dispersed in an Al—Si solid solution because Sn is hardly solved in Al. FIG. 13 schematically shows a metallographic structure of a coating for use in bearing of the Al—Sn—Si alloy formed using an Al—Sn—Si alloy powder by thermal spraying. In FIG. 13, Sn segregated portions resulting from thermal spraying of the fused Al—Sn—Si alloy powder are observed in the metallographic structure. Further, oxides generated through oxidation of the Al—Sn—Si alloy powder are also found in the structure. Such oxide generation occurs because thermal spraying is generally performed while the surface is exposed to the air.
In formation of a bearing coating by means of thermal spraying as described above, because the alloy powder being the thermal spray material is fused to form a coating on the sliding surface of the slide member, low-melting components having been finely dispersed in the alloy powder before thermal spraying, such as Sn or Pb, are segregated while forming a continuous structure during solidification of the fused alloy powder deposited on the sliding surface. The segregated structure has a relatively low mechanical strength due to components of soft metals, such as Sn or Pb, which in turn reduces the strength as the bearing, in particular, causing degradation in fatigue strength when repeated loading is imposed. Further, because thermal spraying is performed in an ordinary atmosphere as noted above, the thermal spray material can undergo oxidation, leading to generation of oxides. In the presence of the oxides generated from the thermal spray material due to oxidation, a section including the oxides is very likely to emerge as a starting point of fatigue failure due to hardness properties of the oxides, which raises a problem in that fatigue performance of the bearing is lowered.